"We'd probably say we're more versatile than we really are, as a band. We're probably the worst people to ask."

I had done the asking, and, in doing so, had offended the Mumford. Not only that, I done so in front of one of his "sons," which is a heavy affront, whether in London or the Blue Ridge Mountains.

Or in Toronto, where Mumford & Sons, the world's biggest British banjo band ever, spoke recently about its long-awaited second album Babel, and along with it, the stunningly sincere single I Will Wait. (The rousing tune is the musical equivalent of Daniel Day-Lewis's "I will come back for you" speech in TheLast of the Mohicans.)

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I had read a review of the song to Marcus Mumford, the quartet's namesake singer and head romantic. He quite liked my paraphrased appraisal – "shouted, triumphant, driving kick drum, earnest lyrics, untouched harmonies, all brought home when the boys jump up an octave and beat the hell out of the chorus" – but looked hurt when I suggested that description of the song could probably be applied to most all of the band's material.

"Beat the hell out of the chorus?" asked Mumford, his accent soft and refined. "Actually, sometimes we beat the hell out of the verses. Sometimes we're quite careful with the chorus."

Sometimes they are. More often they are not, and that's kind of their thing. Mumford & Sons does crescendo music – hallooing, high-minded hill-people stuff of trickling banjos, breaking gallops and cathartic chest-bursting. It sells big, including 2.4 million sold copies of the band's debut LP Sigh No More. The just-out Babel is on track to sell 600,000 copies in its first week, a figure that would tower over the initial numbers of Believe (the summer's previous biggie, from the pop pipsqueak Justin Bieber), which launched at 374,000, according to Nielsen SoundScan.

In 2009 Mumford & Sons broke out from the London folk scene that also spawned Noah and the Whale and Laura Marling. Constant touring since then has grown its audience, to the point where the lads headlined a concert at Toronto's Air Canada Centre a year ago. The rootsy, upswelling pub-rock fills the air of arenas as easily as it does tap rooms, though venue sizes were never a concern. "We would not have imagined playing those big rooms," says acoustic bassist Ted Dwane, an easygoing chap, all leather jacket and blond beard. "Our aspirations were much humbler. We're not that premeditated."

Adds Mumford, a more scholarly fellow, "The songs were designed to fill a musical space in our heads."

Lyrically, the material fills the hearts of young women certainly. Songs typically begin in a lovelorn place, with precious opening lines delivered by kneeling protagonists: "You saw my pain, washed out in the rain," from Ghosts That We Know; "So I was lost, go count the cost," from Holland Road; "Don't let me darken your door," from Reminder; and so on and so forth. These are romance novels set to giant, sweeping roots-rock. Where a Canadian band such as Elliott Brood is inspired by Cormac McCarthy, the Mumford's blockbuster bluegrass is more Harlequin-sponsored.

Mumford tells me the stories are not aesthetic concoctions, but autobiographical reactions. "The road has bled its way into this record," he says, not specifically mentioning I Will Wait and its theme of shamed betrayal.

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The road and touring – "these days of dust, which we've known" – led the band to Nashville, where some of the material was written in a farmhouse. There's a little more polish to this follow-up; brass parts here and there add a more august nature. Mostly, though, the band hasn't changed the formula too much, with Sigh No More producer Markus Dravs (Coldplay, Arcade Fire) back on board. "He's kind of perfect for us," says Dwane, fingering a pack of loose tobacco. To which Mumford adds, "For this record we definitively thought Markus was the one for us."

Who could blame them for keeping the team the same? Formulas worked for Einstein and Coca-Cola, and there is no need yet for any new-styled Mumfords. More of the same is fine, no explanation or justification required.

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